1. Field of the Invention
This invention relates to a composition and method for reducing styrene emissions from unsaturated polyester resins during cure, while maintaining good interlaminate adhesion, particularly in resins made with dicyclopentadiene (DCPD), or blends containing such resins.
2. Description of the Prior Art
Unsaturated polyester resins used to make molded articles are typically the reaction products of dicarboxylic acids and glycols, blended with an ethylenically unsaturated monomer such as styrene. These resins are cured by the addition of an initiator which decomposes to generate free radicals, which in turn activate the cross linking of the styrene monomer with reactive unsaturated groups in the polyester. From the time that such resins are supplied to a mold, until they are thoroughly cured, styrene vapors are emitted from the resin.
It is known that the amount of styrene emitted from the polyester as a resin during cure can be reduced by the addition of small amounts of paraffins to the resin. The paraffins reduce styrene emissions by forming a continuous film on the surface of the resin. The extent of the reduction of styrene emission depends upon the type and level of paraffin used, as well as the resin composition and curing characteristics. It is often possible to reduce styrene emissions by 80 to 90%.
The addition of paraffins to unsaturated polyesters, however, often substantially diminishes the adhesion between successive laminate layers, thus reducing the strength of molded articles made by multilaminate construction. It is known that the addition of adhesion promoters to the resin can offset the reduction in interlaminate adhesion.
Adhesion promoters which have been used include surface active agents which have chemical structures which contain a hydrocarbon chain segment and a relatively more polar segment which includes aromatic and hydroxyl functionalities. An example of such a compound is a bis-hydroxyl bis-ester, which is the reaction product of a fatty acid with bisphenol A diglycidyl ether, having the following structure: ##STR1## wherein p varies from about 5 to 30.
A distinct type of unsaturated polyester is formed by reacting maleic anhydride with one or more glycols, or optionally with water, and reacting the resultant carboxylic acd end groups in an addition reaction with DCPD. These resins, which are often referred to as DCPD resins, can be used as molding resins per se, or in blends with conventional unsaturated polyesters, and have the advantage of curing with reduced shrinkage, thereby giving smooth laminate surfaces.
While the styrene emission levels of DCPD resins can be reduced by the addition of paraffins, the addition of commercially available blends of paraffins and interlaminate bond promoters such as compound (I) yield resins which permit relatively high levels of styrene upon curing.
Public awareness for the need to reduce volatile organic compound (VOC) emissions from polyester resin operations is being voiced by various governmental bodies proposing more stringent controls. The South Coast Air Quality Management Board of El Monte, Calif., were particularly concerned with styrene emmissions in their Staff Report on Jan. 23, 1987, which discussed proposed rules to control such emissions.
Heilmann et al, "Styrene Emission Control Without Interlaminate Adhesion Loss--A New Additive for Unsaturated Polyester", 41st Annual Conference, Reinforced Plastics/Composites Institute, The Society of The Plastics Industry, Inc., Jan. 27-31, 1986, disclose a low styrene emission (LSE) agent identified as "BYK LP-X-5500" which is referred to as a combination of a specific paraffin with a bonding and dispersing agent.
A brochure published by BYK Chemie USA, Wallingford, Conn., U.S.A., "Additive for Reducing Styrene Monomer Emission in Unsaturated Polyester and Vinyl Ester Resins", (September 1986) discloses that manufacturers are being increasingly faced with governmental regulations concerning both environmental and worker health hazards. In the unsaturated polyester industry, strict limits on allowable styrene monomer vapor in the workplace have been established in many countries.
The BYK brochure further describes a line of proprietory additives, namely, "BYK-S-740" for reducing styrene monomer emission.
Japanese Kokai 60/49056 discloses a fiber reinforced unsaturated polyester exhibiting low monomer evaporation during storage and resulting in a laminate with high delamination strength.
Netherlands Application 80/3167 relates to suppression of monomer volatility in unsaturated polyesters by addition of C.sub.12 to C.sub.22 alkanoic acid esters of propoxylated phenols, propoxylated bisphenols or hydroxypropyl phthalates.
Nylander in "Development of a Resin System to Reduce Styrene Evaporation", Section 6-B, pages 1-8 (34th Annual Technical Conference, 1979, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc.) describes reducing styrene vapor concentration in workshops by utilizing an orthophthalic/maleic acid resin with fumed silica and various additives to reduce styrene evaporation from resins.
Walewski et al in "Field Testing of Low Styrene Emission (LSE) Polyester Laminating Resins", Session 1-C, pages 1-4 (40th Annual Conference, 1985, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc.) disclose methods for environmental testing of low styrene emission resins and the use of a new resin whose identity is not revealed.
Other patents of interest include U.S. Pat. No. 3,640,977 to Gonzenbach et al; U.S. Pat. No. 4,010,130 to Matsuo et al; U.S. Pat. No. 3,832,314 to Hoh et al; U.S. Pat. No. 3,959,062 to Hoh et al; U.S. Pat. No. 3,941,904 to Hoh et al; U.S. Pat. No. 3,583,936 to Stahl; U.S. Pat. No. 3,911,185 to Wright, Jr.; U.S. Pat. No. 4,072,808 to Lakshmanan et al; U.S. Pat. No. 4,352,749 to Osaka; U.S. Pat. No. 4,360,622 to Tsuchiya et al; U.S. Pat. No. 4,513,130 to Mizui et al; and U.S. Pat. No. 4,625,008 to Fischer.